1. Field of the Invention
The invention relates to a high-voltage insulation for stator windings of electric machines, comprising a component conductor insulation and a major insulation, in which the component conductors are combined to form bars and surrounded by a major insulation which is impregnated with impregnating resin, the major insulation having multilayer strips which contain mica and carrier material and are wound around the bars in a plurality of layers.
2. Discussion of Background
The electrical and mechanical characteristics of the major insulation are decisive in the insulation of the stator winding conductors of electric machines against the grounded laminate stack. In the case of nominal voltages, of a few 100 V up to 30,000 V, the thickness of the major insulation varies in the range from 0.5 to 8 mm in conjunction with a stress of approximately 2 kV/mm. Added to this are the high demands on the insulation, which result from the thermal stress and, in the case of long machines, from thermomechanical expansion forces in the longitudinal direction of the slot. It is therefore necessary for the slot insulation to be applied in a fashion as free from air as possible and firmly joined to the conductor.
In recent years, the production of continuous winding insulations has to a great extent seen the introduction of vacuum impregnation using curable, solvent-free resins in the case of power windings. It permits the use of fine-mica or laminated-mica strips of the most varied type, which can firstly be wound onto the bars in a virtually dry form using a low proportion of binder. For this purpose, mica strips between 20 and 40 mm wide are applied by hand or machine in thicknesses of 0.1 to 0.3 mm overlapping in a precisely prescribed number of layers. Laminated-mica strips are generally processed using two support layers. Layers made from paper, polyester nonwoven or glass fabric are customary. Fine-mica strips can also be used with one support layer (cf. H. Sequenz "Herstellung der Wicklungen elektrischer Machinen" ["Production of the Windings of Electric Machines"], Springer Verlag, Vienna--N.Y., 1973, pages 147 ff.).
Basically for reasons of mechanical strength, mica strips which have a single support layer are wound on in such a way that the mica layer is directed against the conductor. Because of the structure of the mica material, the adhesion of the innermost layer is not very permanent. Given the mechanical and thermal stresses which occur during operation, detachment phenomena occur which reduce the dielectric strength of the major insulation.
The total proportion of mica in the major insulation is reduced if use is made of mica strips covered all along on both sides by supports.